docs: def _load_fofn(cls, fofn):
          '''Returns dictionary of filename -> short name. Value is None
          whenever short name is not provided'''
          filenames = {}
          f = pyfastaq.utils.open_file_read(fofn)
          for line in f:
              fields = line.rstrip().split()
              if len(fields) == 1:
                  filenames[fields[0]] = None
              elif len(fields) == 2:
                  filenames[fields[0]] = fields[1]
              else:
                  raise Error('Error at the following line of file ' + fofn + '. Expected at most 2 fields.\n' + line)
  
          pyfastaq.utils.close(f)
          return filenames